The present invention relates to methods and gas mixtures for patterning aluminum and aluminum alloy layers in the manufacture of microelectronic devices and circuits. More particularly, the present invention relates to dry etching methods and gas mixtures for patterning aluminum and aluminum alloys by reactive ion etching.
Aluminum and aluminum alloys are popular and frequently used materials for the "metallizations" found in large scale integrated circuits. Such metallization is used, for example, as the interconnection layer on integrated circuits to make ohmic contact to the devices formed in the silicon and to connect the devices to bonding pads on the edge of the chips. Aluminum is used because it adheres well to both silicon and silicon dioxide, can be easily vacuum deposited due to its low boiling point and has high conductivity. In addition to pure aluminum, alloys of aluminum are used to form integrated circuit interconnections for different performance-related reasons. For example, small amounts of copper are added to reduce the potential for electro-migration effects where current applied to the device induces transport of the aluminum. Small amounts of silicon also are added to aluminum metallization to minimize the possibility of electrical spiking that can occur in contact holes. In some cases, titanium is added for the same reason. Aluminum metallization layers may be vacuum-deposited onto semiconductor wafers or substrates by, for example, flash evaporation, filament evaporation, electron-beam evaporation, induction evaporation, and sputtering.
In order to form the patterned metallizations, a series of process steps are ordinarily practiced, e.g., depositing a layer of aluminum or aluminum alloy, coating a photoresist film onto the aluminum or aluminum alloy layer, creating in the photoresist film an image of the predetermined required pattern as by exposing selected portions of the photoresist film to light passing through a mask or grating, removing either the exposed or unexposed portions of the photoresist film depending upon the type of resist employed, and removing the aluminum or aluminum alloy layer in the regions not masked by the remaining photoresist film.
Wet etching, including the step of dipping the entire substrate into an etching solution, has been in general use for several years and is still generally satisfactory to the 4 micron (.mu.m) level. With the trend toward smaller and smaller structures, i.e., 1 .mu.m and smaller, with more and more devices per chip, wet etching is yielding to dry processing. Dry etching procedures, exemplified by plasma etching, offer improved resolution by lessening of line shrinkage due, for example, to undercutting. Other advantages of dry etching include less severe resist adherence requirements and relative ease of disposal of etchant byproducts.
Most of the dry etching of aluminum and aluminum alloys was and still is in many cases done in planar plasma, parallel plate etchers with carbon tetrachloride (CCl.sub.4) as the main gas constituent. The gas pressure regime in which these etchers operate is typically from 0.1 to 1 torr. Since the mechanism of planar plasma etching involves primarily a chemical factor and, to a much lesser extent, an ion bombardment factor, a fair amount of lateral etching takes place. Because of this, the clearing of steep sidewalls is not a major problem, but extensive linewidth loss is normally observed. Other problems encountered with planar plasma etching include, for example, low aluminum etch rates, low selectivities to photoresist and SiO.sub.2, and residue formation. Selectivity refers to the rate at which aluminum is desirably etched compared to the rate at which other materials, such as photoresist and SiO.sub.2 are undesirably etched.
By very careful control of the process parameters, including chamber heating to reduce residue formation, successful planar plasma dry etching has been accomplished in the 2-3 .mu.m range, but with critical dimension (CD) shifts of as much as 0.4 .mu.m. However, with the advent of Very Large Scale Integration (VLSI), where aluminum in the 1 .mu.m linewidth range must be etched, the large CD shifts typically obtained in planar plasma etchers become unacceptable.
Because of the limitations of planar plasma etchers, the focus of dry etching has shifted to reactive ion etching (RIE). The main impetus for the use of reactive ion etching technology is the potential for improvement in the anisotropic nature of the etch. Vertically etched layers are essential to preserving line-width integrity and providing increased device density. The added etch control and selectivity with RIE contributes to the quality of integrated circuits and devices. Reactive ion etching, operating in the pressure regime of 0.01 to 0.1 torr, with the wafers placed on a radio frequency powered cathode, combines chemical etching by active species with ion bombardment in a direction normal to the wafer surface to produce highly directional material removal.
Reactive ion etching of aluminum and aluminum alloys with CCl.sub.4, however, has resulted in problems with the thermal stability of the resist, residues from polymer formation, incomplete grain boundary etching and incomplete silicon etching. Many process modifications have been tried including the use of super cooled substrates and inert gas dilutions of CCl.sub.4 with helium or argon, but none of these methods has solved all of the etching problems.
Several investigators have reported good results for the reactive ion etching of aluminum with boron trichloride (BCl.sub.3) as a major component in a gas mixture. When BCl.sub.3 is used as a source of chlorine, polymer-like film formations sometimes associated with the use of halocarbons are not observed. Boron trichloride is also an effective reducing agent for aluminum oxide, and a scavenger of trace amounts of water vapor. Within limits, these properties can result in a more reproducible etch than can be obtained with carbon tetrachloride, for example, since the induction period, i.e., the time prior to the onset of aluminum etching, which is usually very short with BCl.sub.3, is spent mainly in etching native aluminum oxides. Since BCl.sub.3 plasmas etch aluminum relatively slowly, at rates of 400-500 .ANG./min., BCl.sub.3 is not normally used without additives.
Additions of chlorine to BCl.sub.3 plasmas can result in high etch rates, and a good quality aluminum etch if a proper gas ratio is used. Most of the reactant etching species in this type plasma are produced by the added chlorine. For batch etchers, however, this gas mixture may not be the best choice. Too much chlorine in the mixture results in isotropic etching and poor dimensional control, while too little chlorine results in reactant depletion and decreased etch rates, i.e., a large loading effect. It may be possible to tailor the gas ratios to a fixed wafer load, but if variable size loads are to be etched, the etching characteristics may also vary. The addition of Cl.sub.2 to BCl.sub.3, however, also causes native aluminum oxides to etch more slowly, resulting in longer induction periods.
When small amounts of oxygen, usually under 5%, are added to a BCl.sub.3 plasma, aluminum etch rates can increase by as much as a factor of six. The oxygen reacts with BCl.sub.3 in preference to aluminum, producing reactive chlorine species. Use of this plasma, however, results in poor critical dimension control, due to excessive resist loss and lateral etching. Higher fractions of oxygen, e.g. 10% or more, also cause the formation of particulates which are attributed to boron oxides.
The addition of carbon tetrachloride to BCl.sub.3 is another means of introducing chlorine species. This combination results in higher aluminum etch rates with relatively high selectivity to photoresist, but residue formation on the oxide substrate is frequently a problem.
Thus, what is needed is a reactive ion etching process which rapidly etches the aluminum and aluminum alloys used to fabricate the metallizations for microelectronic circuits and devices, which has high selectivity to photoresists and SiO.sub.2 and dimensional control compatible with 1 .mu.m linewidths, but which has minimal loading effects (dependence of etch rate on total surface area to be etched) and short consistent induction periods.